As noted above, a variety of seals have been designed for internal mixers. The packing type of seal is one example and it employs an adjustable packing gland and a shaft seal comprising a wear ring of glass filled Teflon. A shortcoming of this design is the necessity of constant adjustment of the packing gland in order to make up for wear. The gland pressure causes increased friction on the rotor shaft which requires higher driving torque and causes shaft wear.
Another design comprises the face type seals. One version is spring loaded in which the application of the spring load generates pressure on the sealing face which contacts the shaft. As the internal pressure increases, more spring force is required which generates higher friction and heat. This heat, in turn, causes faster seal face deterioration, specifically if polymer seal materials are used. As the seal face wears, the spring force on the seal surface diminishes which allows more compound to leak. When processed compounds are very abrasive, as is true for halogenated thermoplastics and abrasive additives, accelerated wear and seal failures result.
As a variation, some face type seals are loaded with hydraulic or pneumatic cylinders to compensate for wear of the sealing surface. The pressure or load remains constant, it does not decrease with wear as spring loaded seals. In some seal designs, external lubrication is incorporated to reduce friction and wear between the rotating and stationary seal faces.
Several problems that limit the use of this type of seal include lubricant flow into the mixer cavity and leaks from the seal area which contaminate the surrounding area necessitating frequent clean-up. Also, the seal lubricant selected must be compatible with the material being processed. In some instances, operating temperatures may cause degradation of the lubricant. Lastly, some polymeric seal faces contain molybdenum disulfate, graphite or other lubricants and operate without external lubrication. Friction generated temperature causes softening of this seal type which results in deformation under sealing pressure and accelerated wear.
A third variation of the face type seals is the reverse action seal in which the sealing force is generated by the internal pressure of the mixer. The escaping material generates a pressure against a rotating sealing ring which is allowed to float in the axial direction and generate a sealing pressure on the opposing seal higher than the mixer pressure. This type of seal does not require external adjustment or loading to function, but has the same problem as the other face type seals.
Another type of seal is the close clearance, temperature controlled seal. This seal is not externally lubricated and it is applicable to nonabrasive, self-lubricating materials such as polyolefins and the like. The sealing is achieved by temperature control of the gland to solidify the polymer which generates its own seal. The major problem with this type of seal is that only a limited range of materials can be processed. Abrasive additives or corrosive materials wear the gland and shaft rapidly and the increased clearance causes the seal to leak.
Another type is the pumping screw or forwarding screw seal. Seals of this design are utilized in various applications and employ on the rotor shaft spiral grooves which generate a forwarding force and pump the polymer toward the inside of the mixer. The problem associated with this design is that when internal pressure of the mixer decreases below the pumping pressure of the seal spiral, dislodged material is pumped back into the main melt.
Finally, the labyrinth type seal employs an array of concentric grooves in the rotor or in the stationary part with close clearance to provide the sealing function. The principle of this seal arrangement is that leaking material must flow across a closed clearance dam into the adjacent groove. If sufficient stages of labyrinth grooves are provided, material leak could be eliminated or a controlled leak could result. Problems are also associated with this type of seal. First, due to deflection of the rotor shafts, clearances must be relatively large resulting in considerable leak flow which, in turn, necessitates long labyrinth seal. Second, grooves may become filled with solid or degraded material rendering the seals ineffective.
Thus, despite the widespread existence of Banbury and other types of high intensity internal mixers for more than 50 years and the many and varied designs for seals, a structure has not been provided heretofore that is effective in sealing the rotors, particularly in environments that are abrasive or corrosive or both such as the compounding of certain thermoplastic formulations.